Wavelength conversion chip for use in solid-state lighting and method for making same

ABSTRACT

A wavelength conversion chip is formed by depositing a wavelength conversion material layer on a substrate, segmenting the wavelength conversion layer into a plurality of wavelength conversion chips, and then removing the wavelength conversion chips from the substrate. The wavelength conversion of the chips can be increased by thermal annealing or radiation annealing of the wavelength conversion material. Optical coatings or light extraction elements can be fabricated on the wavelength conversion layer.

TECHNICAL FIELD

A chip containing a wavelength conversion material and a method formaking the chip. The chip can be used in conjunction with a lightemitting diode (LED) to convert light emitted by the LED into anotherwavelength.

BACKGROUND

The recent proliferation of solid-state lighting has created a need forhigh performance wavelength conversion materials. The standard approachis to form wavelength conversion materials (e.g. phosphors) using solidstate processing as known in the art. These phosphors are then grounddown to powders in the micron size range and deposited on a surfaceusing a variety of deposition techniques such as settling,encapsulation, and spray coating. Large extended area devices such ascathode ray tubes (CRTs), fluorescent lighting, and plasma displaysrequire a large amount of standard high volume phosphors. Thesephosphors can be acquired for less than $100/Kg for the large extendedarea devices. Though relatively inexpensive, the phosphors generatedusing this method suffer from high levels of dislocations and latticedefects. In addition, the compositional purity is also difficult tomaintain. In the majority of cases, this does not represent a majorproblem because of the reduced excitation levels. It has been shown,however, in accelerated aging studies that very high excitation levelscan degrade the output luminescence of powdered phosphors severely andimpact overall life performance. These levels of high excitation existwithin solid-state lighting applications. This is mainly due to thesmall size and concentrated flux density of the LED die itself. It isalso true that in the case of individual or arrays of LEDs, phosphorusage is measured in milligrams rather than grams. It is apparent thathigher performance wavelength conversion materials are needed and can beafforded for applications like solid-state lighting. The expense ofhigher performance materials can be absorbed and even offset if theproblems associated with the degradation and reduced conversionefficiency of phosphors used in solid-state lighting can be overcome.

Several material characteristics contribute to this degradation and/orloss in efficiency problem such as lattice defects, out-gassing, andcompositional purity. It has been shown that polycrystalline andmono-crystalline phosphor films either grown on a substrate or as singlecrystal boules tend to exhibit much better luminosity and lifecharacteristics than powders. In addition, every phosphor has a thermalquenching level that can degrade the output at the temperatures createdby elevated excitation levels. In the case of powdered phosphors, thiscan be a major issue because the phosphor particles are usually isolatedfrom any reasonable thermal conduction path. At very high excitationlevels the energy associated with less than unity quantum efficiency andStokes shift losses can induce a significant localized thermal risewithin the phosphor particles. The need exists for creation of animproved thermal conduction path for the luminescent material. Also,because the particles are roughly spherical the packing density can besignificantly degraded. This affects the tradeoff between maximumabsorption/conversion of the excitation energy and reabsorption of theemitted light. The scattering created by the use of a powder can reducethe overall light output due to the backscattering and subsequentabsorption of the generated light.

Mueller-Mach et al. in U.S. Pat. No. 6,696,703 disclose the depositionof a thin film phosphor directly on the LED die. However, as-depositedthin film phosphors have relatively poor wavelength conversionefficiency. A high-temperature annealing step is required in order toproperly activate the phosphor. This annealing step can damage thesemiconductor layers of the LED. In addition, the absorptioncross-sections of most thin film phosphors are low, especially for blueand near ultraviolet (UV) excitations typically used within solid-statelighting. It is neither economical nor practical in most cases to createa sufficiently thick layer of luminescent material directly on the LED.Another drawback to depositing a phosphor directly on the LED die isthat a large portion of the light generated within a deposited phosphorlayer can be trapped due to total internal reflectance. The needtherefore exists for a method to utilize high performance phosphorswithin an LED package such that the best phosphor can be usedefficiently (e.g. with sufficient quantity, minimal backscatter, andmaximum light extraction). The need also exists for a method tofabricate high efficiency phosphors without damaging the LEDsemiconductor layers.

Another important aspect of phosphors relates to characterization andoverall device efficiency. Phosphors are typically characterized interms of quantum efficiency and Stokes shift losses. As an example, apowder phosphor layer is deposited on a glass surface and excited. Theluminescence is measured as a function of excitation energy and theresult is usually compared to a standard phosphor of known quantumefficiency. The losses associated with Stokes shift can be subtractedand the result would be the intrinsic quantum efficiency. Severalproblems exist with this method of characterization such asbackscattered light, coating thickness variability and light trapping.In the case of phosphor powders, the majority of the generated light canescape from the phosphor particles due to their substantially sphericalnature and to scattering centers located on or in the material itself.The main problem measuring the efficiency of phosphor powders isbackscattering of the light from thick samples. For deposited phosphorfilms or grown phosphor boules, however, the problem of measuring thephosphor efficiency is affected by light extraction. The majority of thelight generated in the phosphor can be trapped within the materialitself due to total internal reflection. Several approaches have beenused to solve the total internal reflection problem including variousroughening techniques and shaping approaches. In these cases, theoverall efficiency becomes as much a function of the extraction means asthe conversion efficiency. Deposited phosphor films have the addedcomplication of a secondary substrate material with its associatedindices and losses.

Mayer et al. in U.S. Pat. No. 6,565,770 describe thin interferencepigment flakes that can be made on a flexible substrate and thenmechanically removed by flexing the substrate. The dichroic reflectorsdiscussed are used in security enhancement on money and other decorativeoptical effects. The use of luminescent materials is discussed but isrelated to the formation of a particular optical effect such as UVillumination for security markings. No explanation for improving theoutput efficiency of LEDs or other light emitting devices is discussedand no device based on integrating the phosphor layer with theexcitation source to form an efficient solid-state lighting element isdisclosed.

The use of flake-like phosphors is also discussed by Aoki et al. in U.S.Pat. No. 6,667,574 for use in plasma displays, but the patent againlacks any reference to solid-state lighting applications or methods toenhance their output. In addition, the above two applications are verymuch cost driven because of the large areas typically required in makinga plasma display or the marking of money or decorative items. In orderto maximize the performance of these wavelength-converting materialshigh temperature processing is preferred.

Mueller-Mach et al. in U.S. Pat. No. 6,630,691 disclose a thinsingle-crystal phosphor substrate onto which an LED structure isfabricated by epitaxial growth techniques. However, single-crystalphosphor substrates are expensive and finding a single crystal phosphorsubstrate that has the proper lattice match to allow the growth of theLED structure can be difficult.

Ng et al. in US Patent Application No. 20050006659 disclose a planarsheet of a single-crystal phosphor that is placed over the outputsurface of an LED as a portion of a preformed transparent cap. However,single-crystal phosphor sheets must be grown by epitaxial processes orsliced from bulk single crystals of phosphor material. Single crystalphosphor sheets are therefore too expensive for most practicalapplications. Planar sheets of polycrystalline phosphors are notdisclosed in US Patent Application No. 20050006659. Bonding the planarsheet of a single-crystal phosphor directly to the surface of the LED toimprove heat dissipation in the phosphor sheet is also not disclosed.

A need exists to maximize the efficiency of wavelength conversionmaterials within a solid-state lighting application and to improve thethermal conductivity properties of the materials. In addition, a needexits for low-cost phosphors that have light extraction enhancements andthe ability to control the level and type of scatter within the phosphorin order to enhance the overall conversion efficiency.

SUMMARY OF THE INVENTION

The objective of this invention is to produce high performancewavelength conversion chips for solid-state lighting applications. Oneembodiment of this invention relates to a method for forming a highperformance wavelength conversion layer on a substrate, then removingthe wavelength conversion layer from the substrate in a manner thatcreates thin wavelength conversion chips of luminescent material thatcan then be attached intimately to LEDs.

Another embodiment of this invention is a method for forming awavelength conversion chip that optionally includes an optical coatingon at least one surface the chip. Example optical coatings includereflective coating layers that are wavelength dependent or polarizationdependent and photonic bandgap coatings. Also included in this inventionis the incorporation of single or multilayered antireflection coatingson the surface of the wavelength conversion chips in order to reduceFresnel effects on one or more surfaces.

A method for forming a wavelength conversion chip that optionallyincludes one or more vias that are fabricated through the wavelengthconversion chip is another embodiment of this invention. The vias allowfor the attachment of electrical connections to the upper surface of theLED.

Another embodiment of this invention is a method for fabricating awavelength conversion chip that includes optional light extractionelements that allow light to easily exit from the chip, leading toimproved efficiency. The use of structures within or on the wavelengthconversion chip as well as a variety of sub-wavelength optical elementswithin or on the chip can be used to induce controlled scatter ordirectional scatter.

A further embodiment of this invention is a wavelength conversion chipformed by one of the above processes.

Bonding the wavelength conversion chip to the emitting surface of an LEDto form a solid-state light source is another embodiment of thisinvention. Because the wavelength conversion chip is attached directlyto the LED, the ability to conductively cool the wavelength conversionchip is vastly improved over a powder phosphor.

Another embodiment of this invention is a solid-state light source thatincludes a stack of wavelength conversion chips bonded to an LED. Thestack of wavelength conversion chips can each be fabricated from thesame wavelength conversion material or the chips can be fabricated fromdifferent wavelength conversion materials. The stack allows for animproved thermal conduction path from each wavelength conversion chip tothe LED. The LED itself has a heat sink to maintain the LED die at a lowtemperature during operation.

The wavelength conversion chip can be attached to an LED by atransparent and thermally conducting bonding layer. Multiple wavelengthconversion chips can be attached in a stack using multiple transparentbonding layers. Since the wavelength conversion material is no longer alight scattering powder, a further advantage of this approach is thereduction of backscatter of excitation light due to the reduction inscattering centers. It is also possible to form a much more effectiverefractive index match to the LED die itself. Total internal reflectionwithin the LED die can be frustrated and the extraction of photonsnormally trapped within the LED die can be improved. Bonding materialscan include transparent conducting oxides, inorganic glasses and polymermaterials such as epoxies, acrylates, halogenated polymers, sol-gels,silicones, and xerogels.

The fact that the wavelength conversion chips are formed independent ofthe LED die allows for a wide range of phosphor processing methods.Conventional processes such as sol-gel, molecular beam epitaxy (MBE),chemical vapor deposition (CVD), metal-organic chemical vapor deposition(MOCVD), sputtering, electron beam evaporation, laser deposition, andliquid phase epitaxy (LPE) can be used to process phosphors. Also theformation of quantum dot and various other quantum conversion materialsby self assembly, lithography, and nanoimprinting can be realized toprocess phosphors. These wavelength conversion layers can be grown onamorphous, polycrystalline, and single-crystal substrates. Thewavelength conversion layers are usually polycrystalline, althoughsingle-crystal layers can also be fabricated. The process for formingthe luminescent material can be a batch or a continuous process.Advances in low temperature processing even allow deposition on flexibleorganic substrates with post processing possible after removal of thewavelength conversion layer from the low temperature substrate.Separation from the substrate can be accomplished in a variety ofmanners including optical, chemical and mechanical means. More preferredwould be the use of laser degradation of the interface between thesubstrate and wavelength conversion layer in a manner similar to therelease GaN multi-quantum-well (MQW) LEDs from sapphire single crystalwafers. A more preferable method would be the fabrication of awavelength conversion layer on a flexible polymeric or metalliccontinuous web substrate followed by bending the substrate tomechanically release the phosphor layer. The layer can be released as asingle layer and then diced into chips or the layer may be patterneddirectly while on the substrate using a variety of dicing techniquesincluding but not limited to laser scribing, mechanical dicing, andlithography and then released as individual chips.

Another embodiment of this invention is the formation of a release layerbetween the substrate and the deposited wavelength conversion layer.This release layer can be removed to separate the wavelength conversionlayer from the substrate and the release layer.

Because the fabrication of the wavelength conversion chip is independentof the LED fabrication, processes such as annealing, thermal andelectron beam aging, recrystallization, and other processes known in theart can enhance the conversion efficiency of the wavelength conversionmaterial either while the material is on the growth substrate or beforeapplication of the wavelength conversion chip to the LED. The resultingwavelength conversion chip can be homogeneous, graded, or porous innature. Process selection would in part be based on a cost/performancetradeoff. Typical high performance phosphors can cost over $1000/Kg. Thecost of the fabrication process is, however, only a couple of milligramsper die, which allows the use of even more expensive approaches withoutsignificantly impacting part cost. Since wavelength conversionefficiency can be a major loss factor in many applications, extrapennies spent on improved wavelength conversion materials can translateinto reduced device count within a given application.

A further embodiment of this invention is the use of multiplecompositionally distinct luminescent materials that can be combinedeither within each wavelength conversion chip during wavelengthconversion layer deposition or as a stack of chips during application.The combination of quantum converting structures and more conventionalluminescent material is also included in this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the present invention, as well as otherobjects and advantages thereof not enumerated herein, will becomeapparent upon consideration of the following detailed description andaccompanying drawings, wherein:

FIGS. 1A-1E are side cross-sectional views illustrating process stepsneeded to fabricate a wavelength conversion chip.

FIG. 1F is a side cross-sectional view and FIG. 1G is a top plan view ofa wavelength conversion chip made by the process illustrated in FIGS.1A-1E.

FIGS. 2A-2E are side cross-sectional views illustrating process stepsneeded to fabricate a wavelength conversion chip that includes anoptical coating on one surface.

FIG. 2F is a side cross-sectional view and FIG. 2G is a top plan view ofa wavelength conversion chip that includes an optical coating and thatis made by the process illustrated in FIGS. 2A-2E.

FIGS. 3A-3F are side cross-sectional views illustrating process stepsneeded to fabricate a wavelength conversion chip that includes lightextracting elements and a through via.

FIG. 3G is a side cross-sectional view and FIG. 3H is a top plan view ofa wavelength conversion chip that includes light extracting elements anda through via and that is made by the process illustrated in FIGS.3A-3F.

FIGS. 4A and 4B illustrate side cross-sectional views of example lightemitting diodes that can be used with this invention.

FIG. 5A is a side cross-sectional view of a solid-state light sourcethat includes one wavelength conversion chip.

FIG. 5B is a side cross-sectional view showing example light raysemitted by the solid-state light source illustrated in FIG. 5A.

FIG. 6A is a side cross-sectional view of a solid-state light sourcethat includes two wavelength conversion chips.

FIG. 6B is a side cross-sectional view showing example light raysemitted by the solid-state light source illustrated in FIG. 6A.

FIG. 7A is a side cross-sectional view of a solid-state light sourcethat includes three wavelength conversion chips.

FIG. 7B is a side cross-sectional view showing example light raysemitted by the solid-state light source illustrated in FIG. 7A.

FIG. 8A is a side cross-sectional view of a solid-state light sourcethat includes one wavelength conversion chip.

FIG. 8B is a side cross-sectional view of a solid-state light sourcethat includes two wavelength conversion chips.

FIG. 8C is a side cross-sectional view of a solid-state light sourcethat includes three wavelength conversion chips.

FIG. 9A is a side cross-sectional view of a solid-state light sourcethat includes one wavelength conversion chip that has an opticalcoating.

FIG. 9B is a side cross-sectional view showing example light raysemitted by the solid-state light source illustrated in FIG. 9A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be betterunderstood by those skilled in the art by reference to the above listedfigures. The preferred embodiments of this invention illustrated in thefigures are not intended to be exhaustive or to limit the invention tothe precise form disclosed. The figures are chosen to describe or tobest explain the principles of the invention and its applicable andpractical use to thereby enable others skilled in the art to bestutilize the invention. The above listed figures are not drawn to scale.In particular, the thickness dimensions of the LEDs and wavelengthconversion chips are expanded to better illustrate the various internallayers of the devices.

An important solid-state light source is a blue or ultraviolet emittingLED used in conjunction with one or more wavelength conversion materialssuch as phosphors or quantum dots that convert at least some of the blueor ultraviolet light to other wavelengths. For example, combining ayellow phosphor with a blue emitting LED can result in a white lightsource. The yellow phosphor converts a portion of the blue light intoyellow light. Another portion of the blue light bypasses the yellowphosphor. The combination of blue and yellow light appears white to thehuman eye. Alternatively, combining a green phosphor and a red phosphorwith a blue LED can also form a white light source. The green phosphorconverts a first portion of the blue light into green light. The redphosphor converts a second portion of the blue light into green light. Athird portion of the blue light bypasses the green and red phosphors.The combination of blue, green and red light appears white to the humaneye. A third way to produce a white light source is to combine blue,green and red phosphors with an ultraviolet LED. The blue, green and redphosphors convert portions of the ultraviolet light into, respectively,blue, green and red light. The combination of the blue, green and redlight appears white to the human eye.

In a typical solid-state light source, powdered phosphor materials aremixed into a polymer host material and applied to the LED as a thincoating or as a lens-shaped element. This results in wavelengthconversion elements that can degrade under intense illumination and thathave poor thermal conductivity to the LED.

Solid-state light sources that are fabricated by depositing a thin layerof a phosphor directly on the surface of the LED also have significantdeficiencies. It is important to separate the processes for making thewavelength conversion layers from the processes for fabricating the LED.Some steps required for optimizing the wavelength conversion layers,such as thermal annealing, may be incompatible with maintaining theoutput performance of the LED. Forming and optimizing the wavelengthconversion layers separately from the LED fabrication overcomes suchincompatibilities.

One embodiment of this invention is a process for forming highperformance wavelength conversion chips that are fabricated separatelyfrom the LED sources and that can be subsequently bonded onto the lightoutput surfaces of LEDs. The process can be a batch process or acontinuous web process. The resulting wavelength conversion chips aresubstantially planar in order to facilitate bonding to the LEDs. Thelength and width dimensions of the wavelength conversion chips can begreater than, equal to or smaller than the length and width dimensionsof the LEDs onto which the chips will be attached.

The first step in the process for forming wavelength conversion chips isto select a substrate 10, which is shown in a side cross-sectional viewin FIG. 1A. The substrate provides a physical support for the subsequentdeposition of the wavelength conversion layer. Substrate 10 has a bottomsurface 12 and a top surface 14 opposite bottom surface 12. Substrate 10can be a polymeric material or an inorganic material. The material ofsubstrate 10 can be amorphous, polycrystalline or a single-crystal andcan be a heterogeneous material or a homogeneous material. If substrate10 is a polymeric material or a thin flexible metal layer, substrate 10can be a web that allows for a continuous web process. Depending on thelater process steps, the substrate 10 may need to be transparent tolight so that a laser liftoff process can be used to remove anydeposited wavelength conversion layer. If a mechanical or chemicalprocess is used in later steps to remove any deposited wavelengthconversion layer from substrate 10, then substrate 10 does not need tobe transparent. Example transparent polymeric materials include, but arenot limited to, polyethylene and polyethylene terephthalate (PET).Example flexible metals include stainless steel, molybdenum andgraphite. Example transparent inorganic materials include, but are notlimited to, sapphire, silica and silicate glasses. Optionally, the topsurface 14 of substrate 10 may include a release layer (not shown) tofacilitate the removal of any subsequently deposited materials. Therelease layer is removed by thermal, radiation or mechanical means toseparate the wavelength conversion layer from the substrate and therelease layer. Example materials for the release layer include, but arenot limited to, polymer layers, thin metal layer or inorganic materialsthat preferentially absorb light.

The next process step is to deposit a wavelength conversion layer 20 onthe top surface 14 of substrate 10 as illustrated in a sidecross-sectional view in FIG. 1B. The wavelength conversion layer 20 hasa bottom surface 22 in contact with the top surface 14 of substrate 10and a top surface 24.

The wavelength conversion layer 20 is formed from wavelength conversionmaterials. The wavelength conversion materials absorb light in a firstwavelength range and emit light in a second wavelength range, where thelight of a second wavelength range has longer wavelengths than the lightof a first wavelength range. The wavelength conversion materials may be,for example, phosphor materials or quantum dot materials. The wavelengthconversion layer may be formed from two or more different wavelengthconversion materials. The wavelength conversion layer 20 may alsoinclude optically inert host materials for the wavelength conversionmaterials of phosphors or quantum dots. Any optically inert hostmaterial must be transparent to ultraviolet and visible light.

Phosphor materials are typically optical inorganic materials doped withions of lanthanide (rare earth) elements or, alternatively, ions such aschromium, titanium, vanadium, cobalt or neodymium. The lanthanideelements are lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium and lutetium. Optical inorganic materials include,but are not limited to, sapphire (Al₂O₃), gallium arsenide (GaAs),beryllium aluminum oxide (BeAl₂O₄), magnesium fluoride (MgF₂), indiumphosphide (InP), gallium phosphide (GaP), yttrium aluminum garnet (YAGor Y₃Al₅O₁₂), terbium-containing garnet, yttrium-aluminum-lanthanideoxide compounds, yttrium-aluminum-lanthanide-gallium oxide compounds,yttrium oxide (Y₂O₃), calcium or strontium or barium halophosphates(Ca,Sr,Ba)₅(PO₄)₃(Cl,F), the compound CeMgAl₁₁O₁₉, lanthanum phosphate(LaPO₄), lanthanide pentaborate materials ((lanthanide)(Mg,Zn)B₅O₁₀),the compound BaMgAl₁₀O₁₇, the compound SrGa₂S₄, the compounds(Sr,Mg,Ca,Ba)(Ga,Al,In)₂S₄, the compound SrS, the compound ZnS andnitridosilicate. There are several exemplary phosphors that can beexcited at 250 nm or thereabouts. An exemplary red emitting phosphor isY₂O₃:Eu³⁺. An exemplary yellow emitting phosphor is YAG:Ce³⁺. Exemplarygreen emitting phosphors include CeMgAl₁₁O₁₉:Tb³⁺,((lanthanide)PO₄:Ce³⁺,Tb³⁺) and GdMgB₅O₁₀:Ce³⁺,Tb³⁺. Exemplary blueemitting phosphors are BaMgAl₁₀O₁₇:Eu²⁺ and (Sr,Ba,Ca)₅(PO₄)₃Cl:Eu²⁺.For longer wavelength LED excitation in the 400-450 nm wavelength regionor thereabouts, exemplary optical inorganic materials include yttriumaluminum garnet (YAG or Y₃Al₅O₁₂), terbium-containing garnet, yttriumoxide (Y₂O₃), YVO₄, SrGa₂S₄, (Sr,Mg,Ca,Ba)(Ga,Al,In)₂S₄, SrS, andnitridosilicate. Exemplary phosphors for LED excitation in the 400-450nm wavelength region include YAG:Ce³⁺, YAG:Ho³⁺, YAG:Pr³⁺, YAG:Tb³⁺,YAG:Cr³⁺, YAG:Cr⁴⁺, SrGa₂S₄:Eu²⁺, SrGa₂S₄:Ce³⁺, SrS:Eu²⁺ andnitridosilicates doped with Eu²⁺. Other phosphor materials not listedhere are also within the scope of this invention.

Quantum dot materials are small particles of inorganic semiconductorshaving particle sizes less than about 30 nanometers. Exemplary quantumdot materials include, but are not limited to, small particles of CdS,CdSe, ZnSe, InAs, GaAs and GaN. Quantum dot materials can absorb lightat first wavelength and then emit light at a second wavelength, wherethe second wavelength is longer than the first wavelength. Thewavelength of the emitted light depends on the particle size, theparticle surface properties, and the inorganic semiconductor material.

The transparent and optically inert host materials are especially usefulto spatially separate quantum dots. Host materials include polymermaterials and inorganic materials. The polymer materials include, butare not limited to, acrylates, polystyrene, polycarbonate,fluoroacrylates, chlorofluoroacrylates, perfluoroacrylates,fluorophosphinate polymers, fluorinated polyimides,polytetrafluoroethylene, fluorosilicones, sol-gels, epoxies,thermoplastics, thermosetting plastics and silicones. Fluorinatedpolymers are especially useful at ultraviolet wavelengths less than 400nanometers and infrared wavelengths greater than 700 nanometers owing totheir low light absorption in those wavelength ranges. Exemplaryinorganic materials include, but are not limited to, silicon dioxide,optical glasses and chalcogenide glasses.

When the wavelength conversion layer 20 is formed from phosphormaterials, the phosphors can be deposited by a variety of techniques.The techniques include, but are not limited to, chemical vapordeposition (CVD), metal-organic chemical vapor deposition (MOCVD),sputtering, electron beam evaporation, laser deposition, sol-geldeposition, molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE).Preferred techniques include CVD, MOCVD and sputtering. When thewavelength conversion layer is formed from quantum dot materials andinert host materials, deposition techniques include spin coating, doctorblading or tape casting, self assembly, lithography, and nanoimprinting.

The thickness of the wavelength conversion layer 20 can range from about0.1 micron to about 2000 microns. Preferred thicknesses range from about0.1 micron to about 100 microns.

The next process step is an optional step to increase the wavelengthconversion efficiency of the wavelength conversion layer 20 by thermalannealing or radiation annealing. This step is especially important forphosphors, since as-deposited phosphors may have low wavelengthconversion efficiency if the deposited layer is not properly annealed.The increased wavelength conversion step can be any heat treatment orany radiation treatment of the wavelength conversion material in thewavelength conversion layer 20 that anneals the phosphor. Heating thewavelength conversion material in the wavelength conversion layer to,for example, 600 degrees Celsius for one hour can result in thermalannealing of the wavelength conversion material. Appropriate annealingtemperatures and times may vary for different wavelength conversionmaterials. Example radiation annealing treatments include subjecting thewavelength conversion material in the wavelength conversion layer 20 toinfrared, visible or ultraviolet light 30 as shown in FIG. 1C orsubjecting the wavelength conversion material in the wavelengthconversion layer to electron beam, atomic beam or ion beam bombardment.The radiation sources may be pulsed or continuous. The light sources maybe incoherent or coherent (e.g. laser) sources.

The increased wavelength conversion step is illustrated in FIG. 1C tooccur after the deposition of the wavelength conversion layer and beforesegmentation of the layer. However, the increased wavelength conversionstep may also be done later in the process sequence, including after thewavelength conversion layer is removed from the substrate. Doing theincreased wavelength conversion step after the wavelength conversionlayer is removed from the substrate is necessary if the substrate 10cannot withstand high temperature or high radiation processing.

The next process step is to segment the wavelength conversion layer 20into a plurality of wavelength conversion chips 50. Grooves or streets40 are formed through the wavelength conversion layer 20 as shown in aside cross-sectional view in FIG. 1D. The streets 40 are fabricated intwo directions (only one direction is shown) to form a plurality ofwavelength conversion chips 50 that can be square, rectangular or anyother planar geometric shape. The plurality of wavelength conversionchips 50 is, at this stage, still attached to the substrate 10. Thewavelength conversion chips have a top surface 54 and side surfaces 56.The bottom surface 22 of the wavelength conversion layer 20 is stillattached to the top surface 14 of substrate 10. The streets 40 can beformed by techniques that include, but are not limited to, laserscribing, mechanical scribing or optical lithography accompanied by wetchemical etching, sputter etching or ion beam etching.

The final step is to remove the plurality of wavelength conversion chips50 from substrate 10. For example, the plurality of wavelengthconversion chips 50 can be removed by directing a pulsed laser beam 60though substrate 10 to destroy the adhesion of the bottom surface 22 ofthe wavelength conversion layer 20 to the top surface 14 of thesubstrate 10 as shown in a side cross-sectional view in FIG. 1E. Othermethods of removing the plurality of wavelength conversion chips includethermal, radiation, chemical or mechanical means. If the chips arefabricated on a continuous web, mechanically flexing the web willrelease the chips. The removal can be facilitated if the top surface 14of substrate 10 includes a release layer (not shown) that can bedegraded by thermal, radiation or chemical means. The detachedwavelength conversion chips each have an exposed bottom surface 52.

A side cross-sectional view of wavelength conversion chip 50 isillustrated in FIG. 1F. A top plan view of wavelength conversion chip 50is shown in FIG. 1G. The wavelength conversion chip 50 has a bottomsurface 52, a top surface 54 and side surfaces 56.

Another embodiment of this invention is a process shown in FIGS. 2A-2Gfor forming wavelength conversion chips that optionally include anoptical coating on the top surface of the chips. Examples of opticalcoatings include a dichroic mirror, an antireflection coating or areflecting polarizer coating. The elements with the same numericalreference in FIG. 2 as in FIG. 1 are the same elements as in FIG. 1 andhave the same properties as the elements in FIG. 1. Some of the steps ofthe process shown in FIGS. 2A-2G are identical to the process stepsoutlined above in FIGS. 1A-1G. The process can be either a batch processor a continuous web process. The resulting wavelength conversion chipsare substantially planar in order to facilitate bonding to the LEDs. Thelength and width dimensions of the wavelength conversion chips can begreater than, equal to or smaller than the length and width dimensionsof the LEDs onto which the chips will be attached.

The first step in the process for forming wavelength conversion chipsthat include an optical coating is to select a substrate 10, which isshown in a side cross-sectional view in FIG. 2A. Substrate 10 has abottom surface 12 and a top surface 14 opposite bottom surface 12.Substrate 10 can be a polymeric material, a metal layer or an inorganicmaterial. Example substrate materials are described previously.Optionally, the top surface 14 of substrate 10 may include a releaselayer (not shown) to facilitate the removal of any subsequentlydeposited materials. The release layer may be activated by thermal,radiation or mechanical means. Example release layer materials aredescribed previously.

The next process step is to deposit a wavelength conversion layer 20 onthe top surface 14 of substrate 10 as illustrated in a sidecross-sectional view in FIG. 2B. The wavelength conversion layer 20 hasa bottom surface 22 in contact with the top surface 14 of substrate 10and a top surface 24.

The wavelength conversion layer 20 is formed from one or more wavelengthconversion materials. The wavelength conversion materials may be, forexample, phosphor materials or quantum dot materials or a plurality ofsuch materials. The wavelength conversion layer 20 may also includeoptically inert host materials in addition to the phosphors or quantumdots. Any optically inert host material must be transparent toultraviolet and visible light. Examples of phosphor materials, quantumdot materials and inert host materials are described above for FIGS.1A-1G. Deposition methods are also described above.

An optical coating 70 is deposited on the top surface 24 of thewavelength conversion layer 20. The optical coating can be depositedimmediately following the deposition of the wavelength conversion layer20 as shown in the side cross-sectional view in FIG. 2B. However, it iswithin the scope of this invention that the optical coating may bedeposited later in the overall process. For example, the optical coatingmay be deposited after the wavelength conversion layer is segmented intowavelength conversion chips. Example optical coatings include reflectivecoating layers that are wavelength dependent or polarization dependent,photonic bandgap coatings and single or multilayered antireflectioncoatings on the surface of the wavelength conversion chips that reduceFresnel reflection effects on one or more surfaces.

An example wavelength dependent reflective optical coating 70 is adichoic coating that transmits light having a first range of wavelengthsand reflects light having a second range of wavelengths. For example, acoating 70 can be deposited on the surface of a green-emittingwavelength conversion layer 20. The coating 70 can transmit green lightand reflect blue or ultraviolet light. This type of coating is useful ifthe wavelength conversion chip 50 is attached to a blue or ultravioletLED and the optical coating 70 is on the surface of the chip oppositethe LED. In this example, blue or ultraviolet light emitted by the LEDthat is not absorbed during the first pass through the wavelengthconversion layer 20 is reflected back through the wavelength conversionchip to interact with the wavelength conversion layer 20 a second time.This increases the probability that the blue or ultraviolet light willbe absorbed by the wavelength conversion layer and re-emitted as greenlight, resulting in a higher output of green light.

Another example of a wavelength dependent reflective optical coating 70is a reflecting polarizer composed of linear subwavelength opticalelements. The optical coating 70 can transmit light that is polarizedparallel to the linear subwavelength optical elements and can reflectlight that is polarized perpendicular to the linear subwavelengthoptical elements.

An example of a photonic bandgap optical coating 70 is a photonicbandgap structure that provides directionality to light exiting thewavelength conversion chip. For example, the photonic bandgap opticalcoating can be a two-dimensional array of closely spaced verticalpillars on the surface of the wavelength conversion chip. The array ofpillars can inhibit the transmission of light within the plane of thecoating and direct the emitted light preferentially in a directionperpendicular to the plane of the coating.

An example of an antireflection optical coating 70 is a one-quarterwavelength thick layer of a low refractive index material such asmagnesium fluoride or silicon dioxide. When the optical coating 70 has alower refractive index than the wavelength conversion layer 20 and isone-quarter of an optical wavelength thick, the coating can reduceFresnel reflections from the surface of the wavelength conversion chip.

The next process step is an optional step to increase the wavelengthconversion of the wavelength conversion material in the wavelengthconversion layer 20 by thermal annealing or radiation annealing of thewavelength conversion material. This step is especially important forphosphors, since as-deposited phosphors may have low wavelengthconversion efficiency if the deposited layers are not properly annealed.The increased wavelength conversion step can be any heat treatment orany radiation treatment of the wavelength conversion material in thewavelength conversion layer 20 that anneals the phosphor. Heating thewavelength conversion material in the wavelength conversion layer to,for example, 600 degrees Celsius for one hour can result in thermalannealing of the wavelength conversion material. Appropriate annealingtemperatures and times may vary for different wavelength conversionmaterials. Example radiation treatments include subjecting thewavelength conversion material in the wavelength conversion layer 20 andthe optical coating 70 to infrared, visible or ultraviolet light 30 asshown in FIG. 2C or subjecting the wavelength conversion material in thewavelength conversion layer 20 and the optical coating 70 to electronbeam, atomic beam or ion beam bombardment. The radiation sources may bepulsed or continuous. The light sources may be incoherent or coherent(e.g. laser) sources. As described previously, the process step ofincreasing the wavelength conversion can be done later in the processsequence. For example, the increased wavelength conversion step can bedone after the wavelength conversion chips are removed from thesubstrate 10.

The next process step is to segment the wavelength conversion layer 20and the optical coating 70 into a plurality of wavelength conversionchips 50. Grooves or streets 40 are formed through the optical coating70 and the wavelength conversion layer 20 as shown in a sidecross-sectional view in FIG. 2D. The streets 40 are fabricated in twodirections (only one direction is shown) to form a plurality ofwavelength conversion chips 50 that can be square, rectangular or anyother planar geometric shape. The wavelength conversion chips are, atthis stage, still attached to the substrate 10. The wavelengthconversion chips have a top surface 54 and side surfaces 56. The bottomsurface 22 of the wavelength conversion layer 20 is still attached tothe top surface 14 of substrate 10. The streets 40 can be formed bytechniques that include, but are not limited to, laser scribing,mechanical scribing or optical lithography accompanied by wet chemicaletching, sputter etching or ion beam etching.

The final step is to remove the plurality of wavelength conversion chips50 from substrate 10. For example, the wavelength conversion chips 50can be removed by directing a pulsed laser beam 60 though substrate 10to destroy the adhesion of the chips to the substrate as shown in a sidecross-sectional view in FIG. 2E. Other methods of removing thewavelength conversion chips include thermal, radiation, chemical ormechanical means. The removal can be facilitated if the top surface 14of substrate 10 includes a release layer (not shown) that can bedegraded by thermal, radiation or chemical means.

A side cross-sectional view of wavelength conversion chip 50 isillustrated in FIG. 2F. A top plan view of wavelength conversion chip 50is shown in FIG. 2G. The wavelength conversion chip 50 has a bottomsurface 52, a top surface 54 and side surfaces 56.

Another embodiment of this invention is a process shown in FIGS. 3A-3Hfor forming wavelength conversion chips that optionally include viasthat extend through the wavelength conversion chip or that optionallyinclude arrays of light extracting elements. The elements with the samenumerical reference in FIG. 3 as in FIG. 1 are the same elements as inFIG. 1 and have the same properties as the elements in FIG. 1. Some ofthe steps of the process shown in FIGS. 3A-3H are identical to theprocess steps outlined in FIGS. 1A-1G. As in the previously disclosedprocesses, the process that optionally includes vias or light extractingelements can be either a batch process or a continuous web process.

The first step in the process for forming wavelength conversion chips isto select a substrate 10, which is shown in a side cross-sectional viewin FIG. 3A. Substrate 10 has a bottom surface 12 and a top surface 14opposite bottom surface 12. Substrate 10 can be a polymeric material, ametal layer or an inorganic material. Example substrate materials aredescribed previously. Optionally, the top surface 14 of substrate 10 mayinclude a release layer (not shown) to facilitate the removal of anysubsequently deposited materials. The release layer may be activated bythermal, radiation or mechanical means. Example release layer materialsare described previously.

The next process step is to deposit a wavelength conversion layer 20 onthe top surface 14 of substrate 10 as illustrated in a sidecross-sectional view in FIG. 2B. The wavelength conversion layer 20 hasa bottom surface 22 in contact with the top surface 14 of substrate 10and a top surface 24.

The wavelength conversion layer 20 is formed from one or more wavelengthconversion materials. The wavelength conversion materials may be, forexample, phosphor materials or quantum dot materials or a plurality ofsuch materials. The wavelength conversion layer 20 may also includeoptically inert host materials in addition to the phosphors or quantumdots. Any optically inert host material must be transparent toultraviolet and visible light. Examples of phosphor materials, quantumdot materials and inert host materials are described above for FIGS.1A-1G. Deposition methods are also listed previously.

The next process step is an optional step to increase the wavelengthconversion of the wavelength conversion material in the wavelengthconversion layer 20 by thermal annealing or radiation annealing of thewavelength conversion material. The increased wavelength conversion stepcan be any heat treatment or any radiation treatment of the wavelengthconversion material in the wavelength conversion layer 20 that annealsthe phosphor. Heating the wavelength conversion material in thewavelength conversion layer to, for example, 600 degrees Celsius for onehour can result in thermal annealing of the wavelength conversionmaterial. Appropriate annealing temperatures and times may vary fordifferent wavelength conversion materials. Example radiation treatmentsinclude subjecting the wavelength conversion material in the wavelengthconversion layer 20 to infrared, visible or ultraviolet light 30 asshown in FIG. 3C or subjecting the wavelength conversion material in thewavelength conversion layer 20 to electron beam, atomic beam or ion beambombardment. The radiation sources may be pulsed or continuous. Thelight sources may be incoherent or coherent (e.g. laser) sources. Asdescribed previously, the increased wavelength conversion step can bedone later in the process sequence. For example, the increasedwavelength conversion step can be done after the wavelength conversionchips are removed from the substrate 10.

The next process step is to segment the wavelength conversion layer 20into a plurality of wavelength conversion chips 50. Grooves or streets40 are formed through the wavelength conversion layer 20 as shown in aside cross-sectional view in FIG. 3D. The streets 40 are fabricated intwo directions (only one direction is shown) to form a plurality ofwavelength conversion chips 50 that can be square, rectangular or anyother planar geometric shape. The wavelength conversion chips 50 are, atthis stage, still attached to the substrate 10. The wavelengthconversion chips have a top surface 54 and side surfaces 56. The bottomsurface 22 of the wavelength conversion layer 20 is still attached tothe top surface 14 of substrate 10. The streets 40 can be formed bytechniques that include, but are not limited to, laser scribing,mechanical scribing or optical lithography accompanied by wet chemicaletching, sputter etching or ion beam etching.

The next step is to form one or more vias and/or one or more lightextracting elements in the wavelength conversion chips 50. Via 82 isillustrated in FIGS. 3E-3H. Via 82 is interior to the edges 56 of thewavelength conversion chip 50 and passes through the wavelengthconversion chip 50 from side 52 to side 54 as shown in FIGS. 3F and 3G.Vias are required if one wishes to form electrical pathways through thechip. The vias may be air-filled vias or the vias may be filled with anelectrically conducting material (not shown) such as a metal.

Light extracting elements 80 are elements that aid in the removal oflight from the wavelength conversion chip 50. In general, the refractiveindex of the wavelength conversion layer 20 is higher than therefractive index of air. A portion of the light that is emitted by thewavelength conversion material can be trapped inside the wavelengthconversion chip 50 by total internal reflection. Light extractingelements 80 cause a larger portion of the emitted light to exit thechip. Examples of light extracting elements 80 include, but are notlimited to, vertical holes, conical-shaped holes, holes with polygonalcross-sections, conical-shaped bumps, hemispherical-shaped bumps, bumpswith polygonal cross-sections such as pyramids, grooves, ridges andsubwavelength optical elements. Other examples of light extractingelements are arrays of subwavelength optical elements such as holes orgrooves that are arranged to form photonic crystals. For illustrativepurposes, the light extracting elements shown in FIGS. 3E-3H areconical-shaped holes.

The light extracting elements 80 may be formed on the top surface 54 ofthe wavelength conversion chip 50 and may extend either part way or allthe way through the chip. If the light extracting elements aresubwavelength optical elements, the light extracting elements may bepositioned either on the top surface 54 of the chip or may be fabricatedinside the chip.

Vias 82 and light extracting elements 80 may be formed by techniquesthat include, but are not limited to, laser ablation, mechanical etchingor optical lithography accompanied by wet chemical etching, sputteretching or ion beam etching.

Vias 82 and light extracting elements 80 are shown in FIG. 3E to befabricated in a process step that occurs after the segmentation of thewavelength conversion layer 20 into wavelength conversion chips.However, it is within the scope of this invention that the fabricationof vias and light extracting elements may occur before or at the sametime as the segmentation of the wavelength conversion layer.

The final step is to remove the plurality of wavelength conversion chips50 from substrate 10. For example, the wavelength conversion chips 50can be removed by directing a pulsed laser beam 60 though substrate 10to destroy the adhesion of the chips to the substrate as shown in a sidecross-sectional view in FIG. 3F. Other methods of removing thewavelength conversion chips include thermal, radiation, chemical ormechanical means. The removal can be facilitated if the top surface 14of substrate 10 includes a release layer (not shown) that can bedegraded by thermal, radiation or chemical means.

A side cross-sectional view of wavelength conversion chip 50 isillustrated in FIG. 3G. A top plan view of wavelength conversion chip 50is shown in FIG. 3H. The wavelength conversion chip 50 has a bottomsurface 52, a top surface 54, side surfaces 56, light extractionelements 80 and via 82.

As stated previously, wavelength conversion chips 50 can be used inconjunction with light emitting diodes to form solid-state lightsources. Two examples of suitable light emitting diode structures areillustrated in FIGS. 4A and 4B. Other types of light emitting diodestructures can also be used with this invention.

Light emitting diodes can be constructed from inorganic or organicmaterials. Inorganic light-emitting diodes can be fabricated fromGaN-based semiconductor materials containing gallium nitride (GaN),aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) andaluminum indium gallium nitride (AlInGaN). Other appropriate LEDmaterials include, for example, aluminum nitride (AlN), indium nitride(InN), aluminum gallium indium phosphide (AlGaInP), gallium arsenide(GaAs), indium gallium arsenide (InGaAs), indium gallium arsenidephosphide (InGaAsP), diamond or zinc oxide (ZnO), for example, but arenot limited to such materials. Especially important LEDs for thisinvention are inorganic GaN-based LEDs or ZnO-based LEDs that emit lightin the ultraviolet and blue regions of the optical spectrum.

FIG. 4A is a side cross sectional view of example LED 100. LED 100includes a first reflecting electrode 102, a multi-layer semiconductorstructure 104 and a second reflecting electrode 106, which is on theopposite side of the multi-layer semiconductor structure 104 from thefirst reflecting electrode 102. The multi-layer semiconductor structure104 includes a first doped semiconductor layer 108, an active region 110and a second doped semiconductor layer 112, which is on the oppositeside of the active region 110 from the first doped semiconductor layer108.

The first reflecting electrode 102 and the second reflecting electrode106 may be fabricated from reflecting metals. For example, the firstreflecting electrode 102 and the second reflecting electrode 106 may beformed from one or more metals or metal alloys containing, but notlimited to, silver, aluminum, nickel, titanium, chromium, platinum,palladium, rhodium, rhenium, ruthenium and tungsten. Preferred metalsare aluminum and silver.

The first reflecting electrode 102 and the second reflecting electrode106 may also optionally include a thin layer (not shown) of atransparent material in addition to the metal layer. The optionaltransparent layer is located between the metal layer and the multilayersemiconductor structure 104. Having a two-layer reflecting electrodeincreases the reflectivity of the reflecting electrode compared to asingle-layer reflecting electrode. Increasing the reflectivity of thereflecting electrode increases the light extraction of LED 100 and theoverall external quantum efficiency of LED 100. Preferably therefractive index of the optional transparent layer is less than therefractive index of the semiconductor materials. The preferredrefractive index of the optional transparent layer is between 1.05 and2.3. The thickness of the optional transparent layer can be one-quarterwave or thicker than one-quarter wave. A thickness of one wave isdefined as the wavelength in air of the light emitted by the LED dividedby the refractive index of the transparent layer. The preferredthickness of the transparent layer is one-quarter wave or three-quarterwave. The optional transparent layer can be fabricated, for example,from dielectric materials such as silicon dioxide (SiO₂), siliconnitride (Si₃N₄), magnesium fluoride (MgF₂) or from electricallyconducting materials such as transparent conductive oxides. Transparentconductive oxides include, but are not limited to, indium tin oxide,ruthenium oxide, copper-doped indium oxide and aluminum-doped zincoxide.

The multi-layer semiconductor structure 104 of the LED 100 isfabricated, for example, from nitride-based semiconductor materialscontaining gallium nitride (GaN), aluminum gallium nitride (AlGaN),indium gallium nitride (InGaN), aluminum indium gallium nitride(AlInGaN), aluminum nitride (AlN) or indium nitride (InN) oralternatively from ZnO-based semiconductor materials.

The active region 10 of the multi-layer semiconductor structure 104 is ap-n homojunction, a p-n heterojunction, a single quantum well or amultiple quantum well of the appropriate semiconductor material for theLED 100.

LED 100 is assumed for purposes of illustration to be a flip-chip,GaN-based LED. GaN-based LEDs can include the materials GaN, AlGaN,InGaN and AlInGaN. It should be noted, however, that LED 100 may befabricated from any suitable light-emitting semiconductor material suchas the materials listed above and that a flip-chip structure is notrequired. To briefly summarize the important fabrication steps for thisflip-chip, GaN-based, illustrative example, first a multi-layersemiconductor structure 104 is fabricated on a growth substrate (notshown). A second reflecting electrode 106 is deposited onto themulti-layer semiconductor structure opposite the growth substrate,followed by the attachment of a sub-mount (not shown) to the secondreflecting electrode. The structure is inverted (flipped) and a liftoffprocess removes the growth substrate, exposing the surface 128 of themulti-layer semiconductor structure that was originally attached to thegrowth substrate. Finally, a first reflecting electrode 102 is depositedand patterned on the exposed surface 128 of the multi-layersemiconductor structure 104 opposite the second reflecting electrode106.

The details of the structure and fabrication of the illustrative exampleLED 100 will now be described.

The first doped semiconductor layer 108 is an n-doped GaN layer, whichis epitaxially deposited or otherwise conventionally fabricated on agrowth substrate (not shown). The n-doped GaN semiconductor layer has afirst or upper surface 128 and a second or lower surface 126, oppositethe first surface 128.

The active region 110 is a GaN-based p-n heterojunction, which isepitaxially deposited or otherwise conventionally fabricated on thefirst doped semiconductor layer 108. As an illustrative example, theGaN-based p-n heterojunction can be a GaN/AlGaN heterojunction. TheGaN-based p-n heterojunction active region 110 has a first or uppersurface 124, deposited or fabricated on the second surface 126 of thefirst doped semiconductor layer 108, and a second or lower surface 122,opposite the first surface 124.

The second doped semiconductor layer 112 is a p-doped GaN layer, whichis epitaxially deposited or otherwise conventionally fabricated on theactive region 110. The p-doped GaN semiconductor layer has a first orupper surface 120, epitaxially deposited or otherwise fabricated on thesecond surface 122 of the active region 110, and a second or lowersurface 118, opposite the first surface 120.

The second reflecting electrode 106 of LED 100 is silver, which isdeposited or otherwise conventionally fabricated on the lower surface118 of the second doped semiconductor layer 112. The second reflectingelectrode has a first, upper and inner surface 116 and a second, loweror outer surface 114, opposite the first surface 116.

The first reflecting electrode 102 is aluminum, which is deposited orotherwise conventionally fabricated on the first doped semiconductorlayer 108. The first reflecting electrode 102 has a first, inner orlower surface 130, deposited or fabricated on the first surface 128 ofthe first doped semiconductor layer 108, and a second, outer or uppersurface 132, opposite the first surface 130.

The inner surface 130 of the first reflecting electrode 102 is an innerreflecting surface for the multi-layer semiconductor structure 104 ofthe LED 100. The outer surface 132 of the first reflecting electrode 102is an outer reflecting surface for externally incident light directed toLED 100.

The first reflecting electrode 102 only partially covers the surface 128of the first doped semiconductor layer 108. Portions of the surface 128of the first doped semiconductor layer 108, not covered by the firstreflecting electrode 102, are exposed and those exposed portions of thesurface 128 of the first doped semiconductor layer 108 are an output orexit surface for the light emitted by the LED 100. The portion of thesurface 128 not covered by the first reflecting electrode 102 may becovered with light extracting elements (not shown) to improve lightextraction from LED 100.

The light emitting diode 100 has a first reflecting electrode 102, amulti-layer semiconductor structure 104 having a first dopedsemiconductor layer 108, an active region 110 and a second dopedsemiconductor layer 112, and a second reflecting electrode 106.

The active region 110 emits internally generated light in a wavelengthrange when a voltage is applied across the first reflecting electrode102 and the second reflecting electrode 106. The emitting wavelengthrange of the internally generated light can include any opticalwavelength. For an LED having a p-n heterojunction active region 110,the emitting wavelength range typically has a full width ofapproximately 50 nm at the half-maximum points of the wavelength range.For wavelength conversion applications, preferably the emittingwavelength range is between about 250 nm and about 500 nm.

The total thickness of the multi-layer semiconductor structure 104 isusually on the order of a few microns. For example, the total thicknessof the multi-layer semiconductor structure 104 can be two to fivemicrons.

Example light rays 134 and 138 illustrate internally generated lightthat is emitted by the active region 110. Internally generated light ray134 is emitted by active region 110 toward output surface 128 of LED100. Internally generated light ray 134 is directed at an angle 136 thatis greater than the critical angle for output surface 128. Internallygenerated light ray 134 is reflected by total internal reflection and isredirected toward internal reflective surface 116 of the secondreflecting electrode 106.

Internally generated light ray 138 is emitted by active region 110toward outer surface 128 of the first semiconductor layer 108 of LED100. Internally generated light ray 138 is directed at an angle 140 thatis less than the critical angle for outer surface 128. Internallygenerated light ray 138 is transmitted through outer surface 128.

In the illustrative example LED 100, the first doped semiconductor layer108 is an n-doped layer and the second doped semiconductor layer 112 isa p-doped layer. However, the two layers can be reversed. If the firstdoped semiconductor layer 108 is a p-doped layer, then the second dopedsemiconductor layer 112 is an n-doped layer. The two doped semiconductorlayers 108 and 112 will have opposite n and p conductivity types.

It is well known by those skilled in the art that the multi-layersemiconductor structure 104 may include additional layers in order toadjust and improve the operation of the LED 100. For example, a currentspreading layer may be inserted between surface 130 of the firstreflecting electrode 102 and surface 128 the first doped semiconductorlayer 108. Such a current spreading layer will have the sameconductivity type as the first doped semiconductor layer and willimprove the uniformity of current injection across the entire activeregion. In addition, a current spreading layer may be inserted betweensurface 118 of the second doped semiconductor layer and surface 116 ofthe second reflecting electrode 106. The latter current spreading layerwill have the same conductivity type as the second doped semiconductorlayer. As another example, an electron blocking layer may insertedeither between surface 126 of the first doped semiconductor layer 108and surface 124 of the active region 110 or between surface 122 of theactive region 110 and surface 120 of the second doped semiconductorlayer. The electron blocking layer reduces the escape of electrons fromthe active region.

FIG. 4B is a cross sectional view of another example light emittingdiode, LED 150. LED 150 is similar to LED 100 except that LED 150 isconstructed in a flip-chip configuration with both the first reflectingelectrode 154 and the second reflecting electrode 106 located on thesame side of the LED 150. In this embodiment, the first dopedsemiconductor layer 108 has a larger surface area than the active region110 and the second doped semiconductor layer 112. A portion 152 of thefirst doped semiconductor layer 108 will extend away from the activeregion 110 and the second doped semiconductor layer 112 exposing aportion 152 of the second surface 126 of the first doped semiconductorlayer 108. The first reflecting electrode 154 is located on the exposedsecond or inner surface 126 of the first doped semiconductor layer 108adjacent to the action region 110 instead of the first or outer surface128 of the first doped semiconductor layer 108. The first reflectingelectrode 154 has a first or upper surface 158 and a second or lowersurface 156, opposite the first surface 158. The first surface 158 ofthe first reflecting electrode 154 is deposited or fabricated on theexposed second surface 126 of the first doped semiconductor layer 108.

The first reflecting electrode 154 is in electrical contact with thefirst doped semiconductor layer 108. The first doped semiconductor layer108 functions as a current spreading layer that directs electricalcurrent from the first reflecting electrode 154 to the active region110.

The first surface 128 of the first doped semiconductor layer 108 has noreflecting electrode on its surface. Light emitted by the active region110 can exit across the entire area of the first surface 128 of thefirst doped semiconductor layer 108. The entire surface functions as anoutput surface. The surface 128 may also include optional lightextracting elements (not shown) to improve light extraction from LED150. The first reflecting electrode 154, now on the lower side of LED150, can reflect both internally generated light and externally incidentlight.

Solid-state light sources can be constructed by combining an LED withone or more wavelength conversion chips. Some illustrative examples areshown in FIGS. 5-9. FIGS. 5-7 illustrate solid-state light sources thathave one or more wavelength conversion chips that include one or morevias and one or more light extracting elements. The LEDs in FIGS. 5-7each have a top first reflecting electrode. Vias through the wavelengthconversion chips are required in order to make electrical contacts withthe first reflecting electrodes. FIGS. 8A-8C illustrate solid-statelight sources that have one or more wavelength conversion chips thatinclude light extracting elements, but no vias. The LEDs in FIGS. 8A-8Ceach have first reflecting electrodes on the second or bottom surface ofthe first semiconductor layer 108. FIG. 9 illustrates a solid-statelight source that has a wavelength conversion chip that includes anoptical coating on the top surface of the chip. In the latter example,the wavelength conversion chip does not include a via or a lightextracting element.

Another embodiment of this invention is solid-state light source 200shown in a side cross-sectional view in FIG. 5A. Solid-state lightsource 200 includes LED 100, which has a top first reflecting electrode102, wavelength conversion chip 50 and a bonding layer 202. Thewavelength conversion chip 50 includes light extracting elements 80 andvia 82 that extends through the chip.

The LED 100 of the solid-state light source 200 emits light in a firstwavelength range. The wavelength conversion chip 50 converts light of afirst wavelength range into light of a second wavelength range,different than the first wavelength range. The light of a secondwavelength range has longer wavelengths than the light of a firstwavelength range.

In FIG. 5A, the wavelength conversion chip covers the entire surface 128of LED 100 except for the area occupied by via 82. It is also within thescope of this invention that the wavelength conversion chip 50 may havean area that is larger or smaller than the area of the top surface 128of LED 100.

The wavelength conversion chip is attached to LED 100 by a transparentand thermally conducting bonding layer 202. Preferably the refractiveindex of bonding layer 202 is 1.30 or higher. The high refractive indexof the bonding layer frustrates total internal reflection and improvesthe extraction of photons normally trapped within the LED die. Thebonding layer also provides a thermal conduction path from thewavelength conversion chip 50 to LED 100. Heat generated inside thewavelength conversion chip 50 can pass through the bonding layer 202 andthrough LED 100 to a heat sink (not shown) attached to the bottomsurface 114 of the LED.

Example materials for bonding layer 202 include low melting pointtransparent conducting oxides (TCOs), low melting point inorganicglasses and polymers. Example transparent conducting oxides include tinoxide, indium oxide, gallium oxide, indium tin oxide, ruthenium oxide,copper-doped indium oxide and aluminum-doped zinc oxide. Low meltingpoint inorganic glasses include alkali glasses. Polymer materialsinclude epoxies, acrylates, sol-gels, silicones, chlorinated orfluorinated polymers and xerogels. An example fluorinated polymer istetrafluroethylene.

If bonding layer 202 is a TCO or inorganic glass, sputtering, electronbeam evaporation, chemical vapor deposition or metal-organic chemicalvapor deposition can be used to deposit the bonding layer. If bondinglayer 202 is a polymer, spin coating or spray coating can be used todeposit the bonding layer.

Example light rays 204, 206, 208, 210 and 212 in FIG. 5B illustrate theoperation of solid-state light source 200. When an electrical current isapplied through LED 100 via the first reflecting electrode 102 and thesecond reflecting electrode 106, the active region 110 emits internallygenerated light. The wavelength of the internally generated light can beany optical wavelength. Preferably the wavelength of the internallygenerated light is in the wavelength range from about 250 nm to about500 nm. For wavelength conversion applications, usually the wavelengthof the internally generated light is in the ultraviolet (UV) or blueregions of the optical spectrum. For illustrative purposes only, LED 100is shown to emit UV light. For example, LED 100 can emit 360 nm UVlight.

Internally generated UV light ray 204 is emitted by active region 110 ofLED 100. UV light ray 204 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202 and through the wavelength conversion chip 50 withoutbeing converted. UV light ray 204 exits solid-state light source 200through surface 54 of the wavelength conversion chip 50.

Internally generated UV light ray 206 is emitted by active region 110 ofLED 100. UV light ray 206 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202 and into the wavelength conversion chip 50. Wavelengthconversion chip 50 converts UV light ray 206 into blue light ray 208.The conversion of UV light ray 206 to blue light is an illustrativeexample. Depending on the composition of the wavelength conversionlayer, the UV light could be converted instead into, for example, cyan,green, yellow, red or infrared light. Blue light ray 208 passes throughthe remainder of the wavelength conversion chip 50 and exits solid-statelight source 200 through surface 54 of the wavelength conversion chip.

Internally generated UV light ray 210 is emitted by active region 110 ofLED 100. UV light ray 210 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202 and into the wavelength conversion chip 50. Wavelengthconversion chip 50 converts UV light ray 210 into blue light ray 212.Blue light ray 212 passes through the remainder of the wavelengthconversion chip 50, is extracted from the wavelength conversion chip bylight extracting element 80 and exits the solid-state light source 200.Light extraction element 80 improves the light extraction efficiency ofwavelength conversion chip 50 by reducing the probability of totalinternal reflection of light ray 212 at the top surface 54 of thewavelength conversion chip.

The wavelength conversion chip 50 converts UV light rays 206 and 210into blue light. Light ray 204 passes through wavelength conversion chip50 without conversion. Only a portion of the UV light emitted by theactive region of the LED that passes through the wavelength conversionchip is converted into blue light.

Another embodiment of this invention is solid-state light source 220shown in a side cross-sectional view in FIG. 6A. Solid-state lightsource 220 includes LED 100, which has a top first reflecting electrode102, two wavelength conversion chips 50A and 50B and two bonding layers202A and 202B. The wavelength conversion chip 50A includes lightextracting elements 80A and via 82A that extends through the chip. Thewavelength conversion chip 50B includes light extracting elements 80Band via 82B that extends through the chip.

LED 100 of solid-state light source 220 emits light in a firstwavelength range. Wavelength conversion chip 50A converts light of afirst wavelength range into light of a second wavelength range,different than the first wavelength range. The light of a secondwavelength range has longer wavelengths than the light of a firstwavelength range. Wavelength conversion chip 50B converts light of afirst wavelength range into light of a third wavelength range, differentthan the first wavelength range and the second wavelength range. Thelight of a third wavelength range has a longer wavelength than the lightof a first wavelength range.

In FIG. 6A, the wavelength conversion chips 50A and 50B cover the entiresurface 128 of LED 100 except for the area occupied by vias 82A and 82B.It is also within the scope of this invention that the wavelengthconversion chips 50A and 50B may have an area that is larger or smallerthan the area of the top surface 128 of LED 100.

The wavelength conversion chips are attached to LED 100 by transparentand thermally conducting bonding layers 202A and 202B. Preferably therefractive index of bonding layers 202A and 202B is 1.30 or higher. Thehigh refractive index of the bonding layers 202A and 202B frustratestotal internal reflection and improves the extraction of photonsnormally trapped within the LED die. The bonding layers also provide athermal conduction path from the wavelength conversion chips 50A and 50Bto LED 100. Heat generated inside wavelength conversion chip 50A canpass through the bonding layer 202A and through LED 100 to a heat sink(not shown) attached to the bottom surface 114 of the LED. Heatgenerated inside wavelength conversion chip 50B can pass through thebonding layer 202B, through wavelength conversion chip 50A, throughbonding layer 202A and through LED 100 to a heat sink (not shown)attached to the bottom surface 114 of the LED. Example materials anddeposition methods for bonding layers 202A and 202B have been listedpreviously.

Example light rays 222, 224, 226, 228 and 230 in FIG. 6B illustrate theoperation of solid-state light source 220. When an electrical current isapplied through LED 100 via the first reflecting electrode 102 and thesecond reflecting electrode 106, the active region 110 emits internallygenerated light. The wavelength of the internally generated light can beany optical wavelength. Preferably the wavelength of the internallygenerated light is in the wavelength range from about 250 nm to about500 nm. For illustrative purposes only, LED 100 is shown to emit UVlight. For example, LED 100 can emit 360 nm UV light.

Internally generated UV light ray 222 is emitted by active region 110 ofLED 100. UV light ray 222 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A, through wavelength conversion chip 50A without beingconverted, through bonding layer 202B and through the wavelengthconversion chip 50B without being converted. UV light ray 222 exitssolid-state light source 220 through surface 54B of the wavelengthconversion chip 50B.

Internally generated UV light ray 224 is emitted by active region 110 ofLED 100. UV light ray 224 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A and into the wavelength conversion chip 50A.Wavelength conversion chip 50A converts UV light ray 224 into blue lightray 226. The conversion of UV light ray 224 to blue light is anillustrative example. Depending on the composition of the wavelengthconversion layer, the UV light could be converted instead into, forexample, cyan, green, yellow, red or infrared light. Blue light ray 226passes through the remainder of the wavelength conversion chip 50A,through bonding layer 202B, through wavelength conversion chip 50Bwithout being converted and exits solid-state light source 220 throughsurface 54B of the wavelength conversion chip 50B.

Internally generated UV light ray 228 is emitted by active region 110 ofLED 100. UV light ray 228 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A, through wavelength conversion chip 50A without beingconverted, through bonding layer 202B and into the wavelength conversionchip 50B. Wavelength conversion chip 50B converts UV light ray 228 intogreen light ray 230. Green light ray 230 passes through the remainder ofthe wavelength conversion chip 50B and exits the solid-state lightsource 220 through surface 54B of the wavelength conversion chip 50B.

The wavelength conversion chip 50A converts UV light ray 224 into bluelight. The wavelength conversion chip 50B converts UV light ray 228 intogreen light. Light ray 222 passes through wavelength conversion chips50A and 50B without conversion. Only a portion of the UV light passingthrough the wavelength conversion chips is converted into blue or greenlight. The wavelength conversion chip 50B does not convert the alreadyconverted blue light ray 226 from the wavelength conversion chip 50A.The blue light ray 226 is transmitted through the wavelength conversionchip 50B without being converted.

Another embodiment of this invention is solid-state light source 240shown in a side cross-sectional view in FIG. 7A. Solid-state lightsource 240 includes LED 100, which has a top first reflecting electrode102, three wavelength conversion chips 50A, 50B and 50C and threebonding layers 202A, 202B and 202C. The wavelength conversion chip 50Aincludes light extracting elements 80A and via 82A that extends throughthe chip. The wavelength conversion chip 50B includes light extractingelements 80B and via 82B that extends through the chip. The wavelengthconversion chip 50C includes light extracting elements 80C and via 82Cthat extends through the chip.

LED 100 of solid-state light source 240 emits light in a firstwavelength range. Wavelength conversion chip 50A converts light of afirst wavelength range into light of a second wavelength range,different than the first wavelength range. The light of a secondwavelength range has longer wavelengths than the light of a firstwavelength range. Wavelength conversion chip 50B converts light of afirst wavelength range into light of a third wavelength range, differentthan the first wavelength range and the second wavelength range. Thelight of a third wavelength range has longer wavelengths than the lightof a first wavelength range. Wavelength conversion chip 50C convertslight of a first wavelength range into light of a fourth wavelengthrange, different than the first wavelength range and the secondwavelength range and the third wavelength range. The light of a fourthwavelength range has longer wavelengths than the light of a firstwavelength range.

In FIG. 7A, the wavelength conversion chips 50A, 50B and 50C cover theentire surface 128 of LED 100 except for the area occupied by vias 82A,82B and 82C. It is also within the scope of this invention that thewavelength conversion chips 50A, 50B and 50C may have an area that islarger or smaller than the area of the top surface 128 of LED 100.

The wavelength conversion chips are attached to LED 100 by transparentand thermally conducting bonding layers 202A, 202B and 202C. Preferablythe refractive index of bonding layers 202A, 202B and 202C is 1.30 orhigher. The high refractive index of the bonding layers 202A, 202B and202C frustrates total internal reflection and improves the extraction ofphotons normally trapped within the LED die. The bonding layers alsoprovide a thermal conduction path from the wavelength conversion chips50A, 50B and 50C to LED 100. Heat generated inside wavelength conversionchip 50A can pass through the bonding layer 202A and through LED 100 toa heat sink (not shown) attached to the bottom surface 114 of the LED.Heat generated inside wavelength conversion chip 50B can pass throughthe bonding layer 202B, through wavelength conversion chip 50A, throughbonding layer 202A and through LED 100 to a heat sink (not shown)attached to the bottom surface 114 of the LED. Heat generated insidewavelength conversion chip 50C can pass through the bonding layer 202C,through the wavelength conversion chip 50B, through the bonding layer202B, through wavelength conversion chip 50A, through bonding layer 202Aand through LED 100 to a heat sink (not shown) attached to the bottomsurface 114 of the LED. Example materials and deposition methods forbonding layers 202A, 202B and 202C have been listed previously.

Example light rays 242, 244, 246, 248, 250, 252 and 254 illustrate theoperation of solid-state light source 240. When an electrical current isapplied through LED 100 via the first reflecting electrode 102 and thesecond reflecting electrode 106, the active region 110 emits internallygenerated light. The wavelength of the internally generated light can beany optical wavelength. Preferably the wavelength of the internallygenerated light is in the wavelength range from about 250 nm to about500 nm. For illustrative purposes only, LED 100 is shown to emit UVlight. For example, LED 100 can emit 360 nm UV light.

Internally generated UV light ray 242 is emitted by active region 110 ofLED 100. UV light ray 242 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A, through wavelength conversion chip 50A without beingconverted, through bonding layer 202B, through the wavelength conversionchip 50B without being converted, through bonding layer 202C and throughwavelength conversion chip 50C without being converted. UV light ray 242exits solid-state light source 240 through surface 54C of the wavelengthconversion chip 50C.

Internally generated UV light ray 244 is emitted by active region 110 ofLED 100. UV light ray 224 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A and into the wavelength conversion chip 50A.Wavelength conversion chip 50A converts UV light ray 244 into blue lightray 246. The conversion of UV light ray 244 to blue light is anillustrative example. Depending on the composition of the wavelengthconversion layer, the UV light could be converted instead into, forexample, cyan, green, yellow, red or infrared light. Blue light ray 246passes through the remainder of the wavelength conversion chip 50A,through bonding layer 202B, through wavelength conversion chip 50Bwithout being converted, through bonding layer 202C, through wavelengthconversion chip 50C without being converted and exits solid-state lightsource 240 through surface 54C of the wavelength conversion chip 50C.

Internally generated UV light ray 248 is emitted by active region 110 ofLED 100. UV light ray 248 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A, through wavelength conversion chip 50A without beingconverted, through bonding layer 202B and into the wavelength conversionchip 50B. Wavelength conversion chip 50B converts UV light ray 248 intogreen light ray 250. Green light ray 250 passes through the remainder ofthe wavelength conversion chip 50B, through bonding layer 202C, throughwavelength conversion chip 50C without being converted and exits thesolid-state light source 240 through surface 54C of the wavelengthconversion chip 50C.

Internally generated UV light ray 252 is emitted by active region 110 ofLED 100. UV light ray 252 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202A, through wavelength conversion chip 50A without beingconverted, through bonding layer 202B, through wavelength conversionchip 50B without being converted, through bonding layer 202C and intothe wavelength conversion chip 50C. Wavelength conversion chip 50Cconverts UV light ray 252 into red light ray 254. Red light ray 254passes through the remainder of the wavelength conversion chip 50C andexits the solid-state light source 240 through surface 54C of thewavelength conversion chip 50C.

The wavelength conversion chip 50A converts UV light ray 244 into bluelight. The wavelength conversion chip 50B converts UV light ray 248 intogreen light. The wavelength conversion chip 50C converts UV light ray252 into red light. Light ray 242 passes through wavelength conversionchips 50A, 50B and 50C without conversion. Only a portion of the UVlight passing through the wavelength conversion chips is converted intoblue, green or red light. The wavelength conversion chips 50B and 50C donot convert the already converted blue light ray 246 from the wavelengthconversion chip 50A. The blue light ray 246 is transmitted through thewavelength conversion chips 50B and 50C without being converted. Thewavelength conversion chip 50C does not convert the already convertedgreen light ray 250 from the wavelength conversion chip 50B. The greenlight ray 250 is transmitted through the wavelength conversion chip 50Cwithout being converted.

FIGS. 8A-8C illustrate embodiments of this invention that utilize LED150. LED 150 has the first reflecting electrode 154 positioned on thebottom surface 126 of the first doped semiconductor 108. Since there isno top electrode on LED 150, the wavelength conversion chips do notrequire via holes in order to make electrical contact to the firstreflecting electrode.

Example light rays will not be illustrated in FIGS. 8A-8C since theconversion of UV light, for example, into blue, green or red lightoccurs in the same manner as previously illustrated light rays in FIGS.5, 6 and 7.

Another embodiment of this invention is solid-state light source 300shown in a side cross-sectional view in FIG. 8A. Solid-state lightsource 300 includes LED 150, which has a first reflecting electrode 154positioned on the bottom surface 126 of the first doped semiconductorlayer 108, wavelength conversion chip 50 and a bonding layer 202. Thewavelength conversion chip 50 includes light extracting elements 80 anddoes not include a via.

In FIG. 8A, the wavelength conversion chip covers the entire surface 128of LED 150. It is also within the scope of this invention that thewavelength conversion chip 50 may have an area that is larger or smallerthan the area of the top surface 128 of LED 150.

The wavelength conversion chip is attached to LED 150 by a transparentand thermally conducting bonding layer 202. Preferably the refractiveindex of bonding layer 202 is 1.30 or higher. The high refractive indexof the bonding layer 202 frustrates total internal reflection andimproves the extraction of photons normally trapped within the LED die.The bonding layer also provides a thermal conduction path from thewavelength conversion chip 50 to LED 150. Heat generated inside thewavelength conversion chip 50 can pass through the bonding layer 202 andthrough LED 150 to a heat sink (not shown) attached to the bottomsurface 114 of the LED. Example bonding layer materials have been listedpreviously.

The wavelength conversion chip 50 converts light of a first wavelengthrange emitted by LED 150 into light of a second wavelength range. Forexample, wavelength conversion chip can convert UV light into bluelight.

Another embodiment of this invention is solid-state light source 320shown in a side cross-sectional view in FIG. 8B. Solid-state lightsource 320 includes LED 150, which has a first reflecting electrode 154positioned on the bottom surface 126 of the first doped semiconductorlayer 108, two wavelength conversion chips 50A and 50B and two bondinglayers 202A and 202B. The wavelength conversion chip 50A includes lightextracting elements 80A the wavelength conversion chip 50B includeslight extracting elements 80B.

In FIG. 8B, the wavelength conversion chips 50A and 50B cover the entiresurface 128 of LED 150. It is also within the scope of this inventionthat the wavelength conversion chips 50A and 50B may have an area thatis larger or smaller than the area of the top surface 128 of LED 150.

The wavelength conversion chips are attached to LED 150 by transparentand thermally conducting bonding layers 202A and 202B. Preferably therefractive index of bonding layers 202A and 202B is 1.30 or higher. Thebonding layers also provide a thermal conduction path from thewavelength conversion chips 50A and 50B to LED 150. Heat generatedinside wavelength conversion chip 50A can pass through the bonding layer202A and through LED 150 to a heat sink (not shown) attached to thebottom surface 114 of the LED. Heat generated inside wavelengthconversion chip 50B can pass through the bonding layer 202B, throughwavelength conversion chip 50A, through bonding layer 202A and throughLED 150 to a heat sink (not shown) attached to the bottom surface 114 ofthe LED. Example materials and deposition methods for bonding layers202A and 202B have been listed previously.

As an illustrative example, the wavelength conversion chip 50A convertsUV light into blue light. The wavelength conversion chip 50B converts UVlight into green light.

Another embodiment of this invention is solid-state light source 340shown in a side cross-sectional view in FIG. 8C. Solid-state lightsource 340 includes LED 150, which has a first reflecting electrode 154positioned on the bottom surface 126 of the first doped semiconductorlayer 108, three wavelength conversion chips 50A, 50B and 50C and threebonding layers 202A, 202B and 202C. The wavelength conversion chip 50Aincludes light extracting elements 80A. The wavelength conversion chip50B includes light extracting elements 80B. The wavelength conversionchip 50C includes light extracting elements 80C.

In FIG. 8C, the wavelength conversion chips 50A, 50B and 50C cover theentire surface 128 of LED 150. It is also within the scope of thisinvention that the wavelength conversion chips 50A, 50B and 50C may havean area that is larger or smaller than the area of the top surface 128of LED 150.

The wavelength conversion chips are attached to LED 150 by transparentand thermally conducting bonding layers 202A, 202B and 202C. Preferablythe refractive index of bonding layers 202A, 202B and 202C is 1.30 orhigher. The bonding layers also provide a thermal conduction path fromthe wavelength conversion chips 50A, 50B and 50C to LED 150. Heatgenerated inside wavelength conversion chip 50A can pass through thebonding layer 202A and through LED 150 to a heat sink (not shown)attached to the bottom surface 114 of the LED. Heat generated insidewavelength conversion chip 50B can pass through the bonding layer 202B,through wavelength conversion chip 50A, through bonding layer 202A andthrough LED 150 to a heat sink attached to the bottom surface 114 of theLED. Heat generated inside wavelength conversion chip 50C can passthrough the bonding layer 202C, through the wavelength conversion chip50B, through the bonding layer 202B, through wavelength conversion chip50A, through bonding layer 202A and through LED 150 to a heat sinkattached to the bottom surface 114 of the LED. Example materials anddeposition methods for bonding layers 202A, 202B and 202C have beenlisted previously.

As an illustrative example, the wavelength conversion chip 50A convertsUV light into blue light. The wavelength conversion chip 50B converts UVlight into green light. The wavelength conversion chip 50C converts UVlight into red light.

FIGS. 9A and 9B illustrate side cross-sectional views of anotherembodiment of this invention, solid-state light source 400. Solid-statelight source 400 has a wavelength conversion chip that includes anoptical coating. The optical coating in this example is a dichroicmirror.

Solid-state light source 400 includes LED 150, which has a firstreflecting electrode 154 positioned on the bottom surface 126 of thefirst doped semiconductor layer 108, a wavelength conversion chip 50 anda bonding layer 202. The wavelength conversion chip 50 includes anoptical coating 70, but in this example the wavelength conversion chipdoes not include light extracting elements or a via. However, it iswithin the scope of this invention that the wavelength conversion chipcan optionally include light extracting elements and vias in addition tothe optical coating 70. It is also within the scope of this inventionthat a wavelength conversion chip that has an optical coating may alsobe used with other types of LEDs such as LED 100.

In FIG. 9A, the wavelength conversion chip covers the entire surface 128of LED 150. It is also within the scope of this invention that thewavelength conversion chip 50 may have an area that is larger or smallerthan the area of the top surface 128 of LED 150.

The wavelength conversion chip is attached to LED 150 by a transparentand thermally conducting bonding layer 202. Preferably the refractiveindex of bonding layer 202 is 1.30 or higher. The bonding layer alsoprovides a thermal conduction path from the wavelength conversion chip50 to LED 150. Heat generated inside the wavelength conversion chip 50can pass through the bonding layer 202 and through LED 150 to a heatsink (not shown) attached to the bottom surface 114 of the LED. Examplebonding layer materials have been listed previously.

The wavelength conversion layer 20 of the wavelength conversion chip 50converts light of a first wavelength range emitted by LED 150 into lightof a second wavelength range. For example, the wavelength conversionlayer can convert UV light into green light.

The optical coating 70 is on the top surface 24 of the wavelengthconversion layer 20 and is also on the top surface 54 of the wavelengthconversion chip 50. The optical coating 70 is a dichroic mirror in thisexample. The dichroic mirror transmits light in a first wavelength rangeand reflects light in a second wavelength range. For example, if the LED150 emits UV light and the wavelength conversion chip converts UV lightinto green light, then the dichroic mirror is preferably designed totransmit green light and to reflect UV light. The dichroic mirrorprevents UV light from exiting the solid-state light source 400 andcauses the UV light to be recycled back through the wavelengthconversion layer a second time in order to increase the amount of greenlight produced by the solid-state light source.

Another example (not shown) of an optical coating is a reflectingpolarizer. A reflecting polarizer reflects light of a first polarizationstate and transmits light of a second polarization state. The reflectedlight of a first polarization state is recycled back to LED 150 asexternally incident light.

In cases where a portion of the internally generated light emitted byLED 150 is reflecting by an optical coating and recycled back to LED 150as externally incident light, preferably LED 150 is designed to reflecta significant fraction of the externally incident light. The overallreflectivity of LED 150 to externally incident light depends on severalfactors including the absorption coefficient of the multi-layersemiconductor structure 104, the reflectivity of the first reflectingelectrode 154 and the reflectivity of the second reflecting electrode106. By lowering the absorption coefficient of the multi-layersemiconductor structure, the reflectivity of LED 150 to externallyincident light will increase. Furthermore, increasing the reflectivityof the first reflecting electrode and/or the second reflecting electrodewill increase the reflectivity of LED 150 to externally incident light.

In order to maximize the reflectivity of LED 150 to externally incidentlight, preferably the absorption coefficient (i.e. thethickness-weighted average absorption coefficient of the multiplelayers) of the multi-layer semiconductor structure 104 in the emittingwavelength range of the internally generated light is less than 50 cm⁻¹.More preferably, the absorption coefficient of the multi-layersemiconductor structure in the emitting wavelength range is less than 25cm⁻¹. Most preferably, the absorption coefficient of the multi-layersemiconductor structure in the emitting wavelength range is less than 10cm⁻¹.

Furthermore, in order to improve the reflectivity of LED 150 toexternally incident light, preferably the surface 158 of the firstreflecting electrode 154 has a reflectivity greater than 60 percent inthe emitting wavelength range. More preferably, the surface 158 of thefirst reflecting electrode 154 has a reflectivity greater than 80percent in the emitting wavelength range. Suitable materials for thefirst reflecting electrode that have a reflectivity greater than 80percent include aluminum and silver.

The second reflecting electrode 106 covers a larger surface area thanthe first reflecting electrode 154. Consequently, the reflectivity ofthe second reflecting electrode is more critical than the reflectivityof the first metal electrode. In order to improve the reflectivity ofLED 150 to externally incident light, preferably the reflectivity ofsurface 116 of the second reflecting electrode 106 is greater than 85percent in the emitting wavelength range. More preferably thereflectivity of the surface 116 of the second reflecting electrode isgreater than 90 percent in the emitting wavelength range. Mostpreferably, the reflectivity of the surface 116 of the second reflectingelectrode is greater than 95 percent in the emitting wavelength range. Asuitable material for the second reflecting electrode that has areflectivity greater than 95 percent is silver. In the illustrativeexample for LED 150, the second reflecting electrode is fabricated fromsilver.

The overall reflectivity of LED 150 to externally incident light ispreferably greater than 50 percent. More preferably, the overallreflectivity of LED 150 to externally incident light is greater than 60percent.

When an electrical current is applied through LED 150 via the firstreflecting electrode 154 and the second reflecting electrode 106, theactive region 110 emits internally generated light. The wavelength ofthe internally generated light can be any optical wavelength. Preferablythe wavelength of the internally generated light is in the wavelengthrange from about 250 nm to about 500 nm. For wavelength conversionapplications, usually the wavelength of the internally generated lightis in the ultraviolet (UV) or blue regions of the optical spectrum. Forillustrative purposes only, LED 150 is shown to emit UV light. Forexample, LED 150 can emit 360 nm UV light.

Example light rays 402, 404 and 406 illustrate the operation ofsolid-state light source 400.

Internally generated UV light ray 402 is emitted by active region 110 ofLED 150. UV light ray 402 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 150, throughbonding layer 202 and through the wavelength conversion layer 20 a firsttime without being converted. UV light ray 402 is reflected and recycledback toward LED 150 by optical coating 70, which is designed to reflectUV light. UV light ray 402 is transmitted through the wavelengthconversion layer 20 a second time without being converted, throughbonding layer 202 a second time and reenters LED 150 directed towardsecond reflecting electrode 106. Although in this example, UV light ray402 was not converted during either the first or second passage throughthe wavelength conversion layer, it was possible for UV light ray 402 tobe converted during either transit. The probability of conversion of theUV light ray 402 to, for example, green light depends on the absorptioncoefficient of the wavelength conversion layer to UV light. Increasingthe number of transits by UV light through the wavelength conversionlayer increases the probability of conversion. If UV light ray 402 issubsequently reflected (not shown) by the second reflecting electrode106, UV light ray 402 may pass through the wavelength conversion layer athird time or more than three times.

Internally generated UV light ray 404 is emitted by active region 110 ofLED 150. UV light ray 404 is transmitted through the first dopedsemiconductor layer 108, through top surface 128 of LED 100, throughbonding layer 202 and into the wavelength conversion layer 20 of thewavelength conversion chip 50. Wavelength conversion layer 20 convertsUV light ray 404 into green light ray 406. The conversion of UV lightray 404 to green light is an illustrative example. Depending on thecomposition of the wavelength conversion layer, the UV light couldinstead be converted into, for example, blue, cyan, yellow, red orinfrared light. Green light ray 406 is transmitted through the remainderof the wavelength conversion layer 20, through optical coating 70, whichis a dichroic mirror designed to transmit green light, and exits thewavelength conversion chip 50 and the solid-state light source 400through surface 54.

While the invention has been described with the inclusion of specificembodiments and examples, it is evident to those skilled in the art thatmany alternatives, modifications and variations will be evident in lightof the foregoing descriptions. Accordingly, the invention is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and scope of the appended claims.

1. A method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range comprising the steps of: selecting a substrate; depositing a wavelength conversion layer of at least one wavelength conversion material on said substrate; segmenting said wavelength conversion layer into a plurality of wavelength conversion chips; and removing said plurality of wavelength conversion chips from said substrate by separating said substrate and said plurality of wavelength conversion chips at the interface between said substrate and said plurality of wavelength conversion chips.
 2. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 further comprising the step of: increasing the wavelength conversion in said wavelength conversion layer by thermal annealing or radiation annealing of said wavelength conversion material.
 3. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said segmenting said wavelength conversion layer into said plurality of wavelength conversion chips is by laser scribing, mechanical scribing, or optical lithography accompanied by wet chemical etching, sputter etching or ion beam etching.
 4. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said removing said plurality of wavelength conversion chips from said substrate is by using a pulsed laser beam or using thermal, radiation, chemical or mechanical means.
 5. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 further comprising the step of: depositing an optical coating on one surface of said wavelength conversion layer.
 6. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 further comprising the step of: fabricating one or more vias through said wavelength conversion layer in order to form electrical connections through said wavelength conversion chip.
 7. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 further comprising the step of: fabricating light extracting elements in said wavelength conversion layer in order to improve light extraction from said wavelength conversion chip.
 8. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said wavelength conversion material is polycrystalline.
 9. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said wavelength conversion layer has one or more wavelength conversion materials.
 10. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said wavelength conversion layer is a quantum dot material.
 11. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said depositing a wavelength conversion layer of at least one wavelength conversion material on said substrate is by chemical vapor deposition, metal-organic chemical vapor deposition, sputtering, electron beam evaporation, laser deposition, sol-gel deposition, molecular beam epitaxy, liquid phase epitaxy, spin coating, doctor blading, tape casting, self-assembly, lithography or nanoimprinting.
 12. The method for forming wavelength conversion chips which convert light of a first wavelength range into light of a second wavelength range of claim 1 wherein said plurality of wavelength conversion chips is a plurality of first wavelength conversion chips further comprising the step of: bonding at least one second wavelength conversion chip on at least one of said first wavelength conversion chip, said second wavelength conversion chip converts light of said first wavelength range from said first conversion chip into light of a third wavelength range, said third wavelength range being different from said second wavelength range.
 13. An article formed by the method of claim
 1. 14. An article formed by the method of claim
 6. 15. An article formed by the method of claim
 7. 16. An article formed by the method of claim
 12. 17. An article formed by the method of claim
 5. 18. The article of claim 17 wherein said optical coating is a dichroic mirror that reflects said light of said first wavelength range and transmits said light of said second wavelength range.
 19. The article of claim 17 wherein said optical coating is a reflecting polarizer that reflects light of a first polarization state and transmits light of a second polarization state.
 20. The article of claim 17 wherein said optical coating is a photonic bandgap layer that preferentially directs light emitted by said wavelength conversion chip in a direction substantially perpendicular to the surface of said optical coating.
 21. The article of claim 17 wherein said optical coating is an antireflection coating 